基于MVVM的Windows Phone 8应用程序绑定中的问题

Question

这是我第一个基于Visual Studio提供的默认模板的第一个Windows8应用程序。问题是当我尝试通过在LoadData（）中创建Observable集合的新实例来分配Items属性值时，数据没有绑定，但是当我使用Items.Add方法将项添加到列表中时，我可以在UI中看到数据。我希望有些人可以解释我的行为，如果我错过了任何非常明显的事情。

namespace Sample.ViewModels 
{ 
    public class MainViewModel : INotifyPropertyChanged 
    { 
     public MainViewModel() 
     { 
      this.Items = new ObservableCollection<ItemViewModel>();      
     } 

     /// <summary> 
     /// A collection for ItemViewModel objects. 
     /// </summary> 
     public ObservableCollection<ItemViewModel> Items { get; private set; } 

     private string _sampleProperty = "Sample Runtime Property Value"; 

     /// <summary> 
     /// Sample ViewModel property; this property is used in the view 
     /// to display its value using a Binding 
     /// </summary> 
     /// <returns></returns> 
     public string SampleProperty 
     { 
      get 
      { 
       return _sampleProperty; 
      } 
      set 
      { 
       if (value != _sampleProperty) 
       { 
        _sampleProperty = value; 
        NotifyPropertyChanged("SampleProperty"); 
       } 
      } 
     } 

     /// <summary> 
     /// Sample property that returns a localized string 
     /// </summary> 
     public string LocalizedSampleProperty 
     { 
      get 
      { 
       return AppResources.SampleProperty; 
      } 
     } 

     public bool IsDataLoaded 
     { 
      get; 
      private set; 
     } 

     /// <summary> 
     /// Creates and adds a few ItemViewModel objects into the Items collection. 
     /// </summary> 
     public void LoadData() 
     { 

      try 
      { 
       using (IQContext context = new IQContext("isostore:/Test.sdf")) 
       { 
        var query = (from c in context.Categories 

           select new ItemViewModel 
           { 
            CategoryId = c.CategoryId, 
            CategoryName = c.CategoryName 

           }); 

        // Items present in the list. 
        this.Items = 
         new ObservableCollection<ItemViewModel>(query); 

        // this.Items.Add(new ItemViewModel() 
        //  { CategoryId = 1, CategoryName = "Rishi"}); // This Works 

        this.IsDataLoaded = true; 
       } 
      } 

      catch (Exception ex) 
      { 
       throw ex; 
      } 
     } 

     public event PropertyChangedEventHandler PropertyChanged; 
     private void NotifyPropertyChanged(String propertyName) 
     { 
      PropertyChangedEventHandler handler = PropertyChanged; 
      if (null != handler) 
      { 
       handler(this, new PropertyChangedEventArgs(propertyName)); 
      } 
     } 
    } 
} 

Answer 1

你Items财产应实现INPC因为你改变LoadData参考和UI需要通知：

public ObservableCollection<ItemViewModel> Items { get; private set; } 

到

private ObservableCollection<ItemViewModel> items; 
public ObservableCollection<ItemViewModel> Items 
{ 
    get 
    { 
     return this.items; 
    } 

    private set 
    { 
     this.items = value; 
     NotifyPropertyChanged("Items"); 
    } 
}